| Budget Amount *help |
¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 1988: ¥400,000 (Direct Cost: ¥400,000)
Fiscal Year 1987: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
The fracture process of most ductile metals generally consists of three stages; i) voids nucleation at the second phase particle sites either due to cracking of the particles of decohesion between the particles and the matrix, ii) the voids growth under the multi-axial stress state, and iii) the voids coalescece leading to the final fracture.
In the present project, the void nucleation and growth process is experimentally studied, based on the real time observation of void density by ultrasonic method and also by the examination of dimple distribution on the fracture surface. In the experiment, tensile fracture test of mild steel was performed. Several kinds of materials were chosen to examine the effect of the concentration of the second phase particles which can be the void initiation sites. The shape of the specimen used in the test was a circular cross-section bar with a circumferential notch. Speci-mens without notch were also used. The size of the notches was changed to obtain dif
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ferent tri-axial stress states. Tensile fracture test was conducted by an instron type testing machine at a quasi-static strain rate. The mode of fracture was a typical cup and corn type. The etched section along the center axis indicates that a small region near the fracture cross-section is highly deformed and the voids are concentrated at a thin layer along the fracture section. Tensile fracture test of ductile mild steel changing the stress triaxiality and impurity concentration has shown the following results by the observation of the dimple distribution on the fracture surface.
1) In case of rather rich impurity material, the number and area densities of the main dimples increase with the stress triaxiality and they approach to limited values.
2) In case of high purity material, the number density of the dimples does not depend on the stress triaxiality. The area density increases with the stress triaxiality.
The real time measurements of the ultrasonic wave attenuation have consistent results with the above, and also revealed that the nucleation and growth process of the voids do not start untill very close to the fracture instant after the development of large plastic deformation. Concerning to the attenuation measurement, the attenuation due to the voids and cleavage have not been distinguished so far, however, the method of changing the frequency might be able to overcome this problem. The above obtained results can qualitatively be explained by assuming a microscopic mechanism of voids nucleation and growth process, which includes the mutual interference of the voids. Less
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